Method and apparatus for measuring fiber twist by polarization tracking

Abstract

A method of measuring fiber twist in a multi-core optical fiber bearing an FBG with polarization dependent reflectivity. The state of polarization of the launched light is adjusted until the reflected FBG wavelength is maximal, indicating that light reaching the FBG is linearly polarized, and the polarization axis of the light reaching the FBG is aligned with the slow birefringent axis of the FBG; the SOP of launched light is now measured. Bending experienced by the fiber is measured conventionally, and birefringence produced by bending of the multi-core optical fiber is calculated. A candidate amount of twist between the launch location and the FBG is proposed, and the corresponding twist-induced birefringence is calculated. When calculations show that light with the launched SOP becomes linearly polarized and aligned with the FBG after traversing a fiber section with the calculated birefringences and proposed rotation, the amount of twist has been properly identified.

What is claimed as new and desired to be protected by Letters Patent of the United States is: 
     
       1. A method of measuring fiber twist in a multi-core optical fiber comprising a center core and a plurality of peripheral cores at a distance radially from said center core and running parallel to said center core, said center core comprising at least one fiber Bragg grating with polarization-dependent reflectivity at a specified distance from a launch end of the multi-core fiber, said multi-core optical fiber being operably located to receive launched light, the launched light comprising a state of polarization (“SOP”) and a polarization axis, the fiber Bragg grating comprising a fast birefringent axis and a slow birefringent axis, said method comprising:
 calibrating intrinsic fiber birefringence in the center core and birefringence produced in the center core by bending and twisting; 
 varying the SOP of the launched light while measuring the wavelength of light reflected from an FBG until a maximum polarization-dependent reflectivity wavelength of the FBG is detected; 
 measuring the SOP of the launched light when a maximum polarization-dependent reflectivity wavelength of the FBG is detected, the launched light entering the multi-core fiber becoming propagated light, wherein while said launched light has said SOP, said propagated light reaching the FBG is linearly polarized thereby having a polarization axis, and the polarization axis of the propagated light at the FBG is aligned with the slow birefringent axis of the FBG; 
 determining a local bending of the multi-core optical fiber based on strain sensed in the plurality of peripheral cores; 
 calculating a bend-induced birefringence produced by the determined bending of the multi-core optical fiber; 
 calculating a derived SOP at the FBG based on the SOP of the propagated light, which evolves over the specified distance in a manner determined by birefringence produced by the determined local bending, the calibrated intrinsic fiber birefringence, and a candidate level of fiber twist; and 
 adjusting the candidate level of fiber twist until the derived SOP at the FBG is linear and aligned with the FBG rotated by the candidate level of fiber twist to derive an estimated level of fiber twist. 
 
     
     
       2. The method according to  claim 1 , wherein said varying the SOP of the launched light until the maximum polarization-dependent reflectivity wavelength of the FBG is detected comprises:
 detecting, using a wavelength measuring device, a maximum instantaneous polarization-dependent reflectivity wavelength. 
 
     
     
       3. The method according to  claim 2 , wherein said varying the SOP of the launched light until the maximum polarization-dependent reflectivity wavelength of the FBG is detected comprises:
 launching the launched light from a light source that emits the launched light over a spectral range that includes a Bragg wavelength of the reflected light reflected by the at least one fiber Bragg grating; 
 measuring the Bragg wavelength of the reflected light; and 
 varying the state of polarization of the launched light until the maximum instantaneous polarization-dependent reflectivity wavelength is detected. 
 
     
     
       4. The method according to  claim 2 , wherein the wavelength measuring device comprises one of a spectrometer, a scanning interferometer, and a tunable narrow-band filter. 
     
     
       5. The method according to  claim 4 , wherein the spectrometer comprises a fiber-coupled CCD spectrometer, the scanning interferometer comprises a scanning Mach-Zehnder interferometer, and the tunable narrow-band filter comprises a tunable fiber Fabry-Perot filter. 
     
     
       6. The method according to  claim 1 , wherein said adjusting the candidate level of fiber twist until the derived SOP at the FBG is linear and aligned with the FBG rotated by the candidate level of fiber twist comprises:
 assigning an error figure-of-merit for how well the candidate level of fiber twist corresponds to a linear state of polarization, which is aligned with the fiber Bragg grating rotated by the candidate level of fiber twist; 
 adjusting the candidate level of fiber twist and noting an associated error figure-of-merit; and 
 repeating said adjusting the candidate level of fiber twist, and noting an error figure-of-merit associated with the adjusted candidate level of fiber twist until a minimum error figure-of-merit is achieved, thereby yielding an estimated fiber twist of the multi-core optical fiber. 
 
     
     
       7. A method of measuring fiber twist in a multi-core optical fiber comprising a center core and a plurality of peripheral cores at a distance radially from said center core and running parallel to said center core, said center core comprising at least one fiber Bragg grating with polarization-dependent reflectivity at a specified distance from a launch end of the multi-core fiber, said multi-core optical fiber, being operably located to receive launched light, the launched light comprising a state of polarization (“SOP”) and a polarization axis, the fiber Bragg grating comprising a fast birefringent axis and a slow birefringent axis, said method comprising:
 calibrating intrinsic fiber birefringence in the center core and birefringence produced in the center core by bending and twisting; 
 varying the SOP of the launched light while measuring the wavelength of light reflected from an FBG until a minimum polarization-dependent reflectivity wavelength of the FBG is detected; 
 measuring the SOP of the launched light when a minimum polarization-dependent reflectivity wavelength of the FBG is detected, the launched light entering the multi-core fiber becoming propagated light, wherein while said launched light has said SOP, said propagated light reaching the FBG is linearly polarized thereby having a polarization axis, and the polarization axis of the propagated light at the FBG is aligned with the fast birefringent axis of the FBG; 
 determining a local bending of the multi-core optical fiber based on strain sensed in the plurality of peripheral cores; 
 calculating a bend-induced birefringence produced by the determined bending of the multi-core optical fiber; 
 calculating a derived SOP at the FBG based on the SOP of the launched light, which evolves over the specified distance in a manner determined by birefringence produced by the determined local bending, the calibrated intrinsic fiber birefringence, and a candidate level of fiber twist; and 
 adjusting the candidate level of fiber twist until the derived SOP at the FBG is linear and aligned with the FBG rotated by the candidate level of fiber twist to derive an estimated level of fiber twist. 
 
     
     
       8. The method according to  claim 7 , wherein said varying the SOP of the launched light while measuring the wavelength of light reflected from an FBG until a minimum polarization-dependent reflectivity wavelength of the FBG is detected comprises:
 detecting, using the wavelength measuring device, the minimum instantaneous polarization-dependent reflectivity wavelength. 
 
     
     
       9. The method according to  claim 8 , wherein said varying the SOP of the launched light while measuring the wavelength of light reflected from an FBG until a minimum polarization-dependent reflectivity wavelength of the FBG is detected comprises:
 launching the launched light from a light source that emits the launched light over a spectral range that includes a Bragg wavelength of the reflected light reflected by the at least one fiber Bragg grating; 
 measuring the Bragg wavelength of the reflected light; and 
 varying the state of polarization of the launched light until the minimum instantaneous polarization-dependent reflectivity wavelength is detected. 
 
     
     
       10. The method according to  claim 8 , wherein the wavelength measuring device comprises one of a spectrometer, a scanning interferometer, and a tunable narrow-band filter. 
     
     
       11. The method according to  claim 10 , wherein the spectrometer comprises a fiber-coupled CCD spectrometer, the scanning interferometer comprises a scanning Mach-Zehnder interferometer, and the tunable narrow-band filter comprises a tunable fiber Fabry-Perot filter. 
     
     
       12. The method according to  claim 7 , wherein said adjusting the candidate level of fiber twist until the derived SOP at the FBG is linear and aligned with the FBG rotated by the candidate level of fiber twist comprises:
 assigning an error figure-of-merit for how well the candidate level of fiber twist corresponds to a linear state of polarization, which is aligned with the fiber Bragg grating rotated by the candidate level of fiber twist; 
 adjusting the candidate level of fiber twist and noting an associated error figure-of-merit; and 
 repeating said adjusting the candidate level of fiber twist, and noting an error figure-of-merit associated with the adjusted candidate level of fiber twist until a minimum error figure-of-merit is achieved, thereby yielding an estimated fiber twist of the multi-core optical fiber. 
 
     
     
       13. The method according to  claim 12 , wherein the at least one fiber Bragg grating comprises a plurality of successive pairs of fiber Bragg gratings, the method further comprising sequentially estimating a fiber twist segment between each successive pair of fiber Bragg gratings to yield a twist geometry of the multi-core optical fiber, a launch state of polarization for a successive pair of fiber Bragg gratings being an end state of polarization for a preceding pair of fiber Bragg gratings.